1. Field of the Invention
The present invention relates to a viewing optical instrument having a roof prism, which is adapted to invert an object image in the vertical and horizontal directions. The present invention also relates to a roof prism.
2. Description of the Related Art
In viewing optical instruments, such as a monocular or binocular telescope, those using a roof prism having a pair of reflection surfaces intersecting at a nominal face angle of 90 degrees can be made small since an object image formed by an objective optical system can be inverted in the vertical and horizontal directions by a single roof prism. However, since the pupil of the objective optical system is split by an edge line of the reflection surfaces, the face angle must be highly precise. If the face angle is not accurate, a viewable double image occurs.
As is well known, in the case where light is reflected by a reflection surface, a phase difference is produced between two orthogonal P-polarized and S-polarized light components of a light wave before and after the reflection takes place. The phase difference has no adverse influence on the image to be viewed in an optical system having a Porro prism or the like in which the pupil is not split. To the contrary, the image to be viewed can be deteriorated in an optical system having a roof prism or the like in which the pupil is split by the edge line, due to wavefront aberrations caused by a difference in the polarization state between the lights emitted from the pair of reflection surfaces. This is because a change in the polarization state due to a phase difference caused by the reflection surfaces of the roof prism differs between the light incident on one of the reflection surfaces (first reflection surface) and emitted from the other reflection surface (second reflection surface), and the light incident on the second reflection surface and emitted from the first reflection surface. The larger the phase difference produced by the reflection surfaces, the greater the difference in the polarization state caused thereby. If the difference in the polarization state becomes large, the wavefront aberrations can increase, and consequently, a double image occurs and is viewed, and the contrast is reduced to the same as a pair of reflection surfaces having an inaccurate face angle.
To reduce the phase difference caused by the reflection surfaces of the roof prism, it is known to coat the reflection surface with a metal layer such as an aluminum or silver layer. It has been confirmed that the metal layer can reduce the phase difference in question. However, it has been found that the metal coating layer cannot sufficiently reduce the phase difference, particularly in accordance with an increase in the machining precision of the roof prism. Namely, if the machining precision is not high enough, it cannot be determined whether the deterioration of the image to be viewed is caused by the face angle error or by the phase difference. Consequently, the quality of the image is evaluated based on a combination of the face angle error and the phase difference. However, if the machining precision is increased, significance is chiefly placed on the phase difference.